The Loch of Stenness is a sea loch and is the deepest loch on the Mainland, it is slightly smaller in area and volume than the Loch of Harray. It is located at 3.25 miles (5.23 kilometres) northeast of Stromness,[2] and is the largest brackish lagoon in the UK.[5] Its outflow is into the Bay of Ireland and the Hoy Sound at Brig o’ Waithe[6] where tidal currents influence the loch but cause little variation in its level. The loch is connected to the Loch of Harray at the Bridge of Brodgar[7] and both lochs together cover an area of 19.3 square kilometres (7.5 square miles)[7] making the two combined the ninth largest loch in Scotland by area (as listed by Murray and Pullar (1910)). The Loch of Stenness has a maximum depth of 5.2 metres (17 ft) and an average depth of 3.2 metres (10 ft).
The Loch of Stenness is adjacent to the World Heritage sites of Ring of Brodgar and the Stones of Stenness. The Watch Stone, a solitary monolith 5.6 m (18 ft) high stands where the loch joins with the Loch of Harray, at Bridge of Brodgar.
The loch and its surrounding area underwent detailed geophysical and multibeam sonar surveys in 2011 and 2012 to investigate the drowned palaeo-landscape. The surveys indicated significant archaeological features in the loch including a circular structure possibly a henge.[10]